Study of pyrite oxidation by cyclic voltammetric, impedance spectroscopic and potential step techniques

Abstract

Pyrite oxidation in chloride solutions was investigated with cyclic voltammetry, a.c. impedance and potential step techniques. The oxidation reactions of pyrite were examined by cyclic voltammetric technique and a two-step reaction with a passivation film forming as a first-step product was proposed. An equivalent circuit was then postulated based on the oxidation reactions. Parameters indicated in the equivalent circuit such as reaction resistance and pseudo-capacitance caused by the passivation film, were determined by a.c. impedance measurements. A mathematical formula was derived from the concept of the equivalent circuit to explain the depression of the semicircle in the complex plane plot. When the semicircle is depressed, the mathematical formula indicates that the reaction resistance should be obtained from the intersection of the semi-circle with Z′-axis instead of the semicircle diameter. Potential step chronoamperometric technique was then applied to measure the charging current, which is caused by the pseudo-capacitance of the passivation film, to examine the proposed equivalent circuit. The peak charging current densities at 1.10 and 0.90 V vs SHE obtained from the equivalent circuit and the a.c. impedance measurements are 110 and 75 mA cm⁻², respectively. They are consistent with the peak current densities of 105 and 69 mA cm⁻² at 1.10 and 0.90 V, respectively, determined by the potential step chronoamperometric measurements.